In these years, many attempts of using a protein solution as droplets have been made. Examples of such attempts include a transmucosal administration as a drug delivery method, and applications to a biochip or a biosensor because they need only a trace amount of protein. In addition, a method of using protein liquid microdroplets has also attracted attention in the field of screening a bioactive substance (Japanese Patent Application Laid-Open No. 2002-355025; Allain L R, et al., “Fresenius J. Anal. Chem.” 2001, Vol. 371, p. 146-150; and Howard E I, Cachau R E “Biotechniques” 2002, Vol. 33, p. 1302-1306).
Recently, protein, particularly, enzyme or useful protein having bioactivity is going to be able to be mass-produced through a gene-recombination technique so that liquid droplets formation of protein can become a useful means for the search and application of protein as new medicine, and the applicable field. Above all, the means of administering various drugs to a patient with the use of the liquid fine droplet has become more important, particularly in the respect of administering protein, peptide and other biological substances through a lung. Lungs have alveoli with a surface area as large as 50 to 140 m2, have epithelium which is an absorption barrier as extremely thin as 0.1 μm, in addition, have enzymatic activity lower than that of the alimentary canal, and accordingly have received attention as a substituting administration route for the injection of a high-molecule-peptide-based drug represented by insulin.
In general, it is known that the intrapulmonary deposition of a liquid fine droplet of a drug largely depends on an aerodynamic particle size thereof, and above all, in order to deliver the droplet to the alveoli, deep parts of the lung, it is indispensable to develop an administration form capable of administering droplets having particle sizes of 1 to 5 μm and the narrow distribution of the particle sizes with high reproducibility and stable formulation.
There have been conventionally some methods of administering a formulation to the interior of the body particularly to a perimeter of a respiratory organ, so that these methods will be now explained with examples. There is a metered-dose inhaler (MDI) of aerosolizing the formulation in a suspensoid aerosol form enables quantitative atomisation, by employing a liquefied incombustible or flame-resistant gas as a pressure carrier, and controlling a unit volume of the liquefied gas to be ejected at a single time. However, the inhaler has a problem that the size of a droplet in the above-described range is not sufficiently controlled, and besides, the pressure carrier may not be good for health. There is also a spraying method used for atomising a liquid formulation, which employs water and ethanol as a medium, and converts the liquid formulation to microdroplets by ejecting it together with a gas under pressure for transportation through a capillary. Accordingly, it is theoretically possible for the atomising method to control a atomised amount by specifying a fluid volume of the liquid formulation supplied into such a capillary flow path, but is difficult to control the diameter of the droplet.
Particularly, a spray type of atomisation uses a gas under pressure which has been used in a process for making microdroplets from liquid, subsequently as a gas for transporting the atomised microdroplets in a flow. For this reason, it is structurally difficult to vary a quantity (density) of the microdroplets floating in airflow for transportation according to a purpose.
As a method of producing the above-described droplets with a narrow particle size distribution, there is a report on the use of a droplet-generating device which forms extremely microdroplets based on a sort of principle used in an ink jet printing (for instance, U.S. Pat. No. 5,894,841 and Japanese Patent Application Laid-Open No. 2002-248171). Here, such ink jet system will be described. The system consists of the procedure of introducing a liquid to be ejected into a small chamber, applying pushing force to the liquid, and ejecting the droplets through an orifice. A usable pushing method includes, for instance, a method of forming bubbles for ejecting the droplets through the orifice on the chamber with the use of an electro-thermal converter such as a thin film resistor (thermal ink jet system), and a method of directly pushing a liquid through the orifice on the chamber with the use of a piezooscillator (piezo ink jet system). The chamber and the orifice are built in a print head element, and the print head element is connected to a supply source for a liquid and also to a controller for controlling the ejection of the droplets.
When making a drug absorbed from lungs, it is necessary to precisely control the dosage particularly for the above-described protein formulation, so that liquid droplet formation based on the principle of the ink jet system is a very preferable form, because it can control an ejection rate. In addition, though a liquid is required to be reliably ejected, a protein solution having only adjusted surface tension and viscosity is ejected unstably, so that there was a case where the protein solution is hardly ejected with a high degree of reproducibility and efficiency.
The liquid droplet formation of the above-described protein and peptide on the basis of a principle of the ink jet system has a problem that the protein has a frail spatial configuration and therefore may cause the aggregation and decomposition of the protein when the configuration has been destroyed. When the liquid droplets are formed based on the principle of the ink jet system, physical force such as pressure and shear force are applied to the liquid droplets, and each liquid fine droplet has its peculiar high surface energy. They make the configuration of most of proteins unstable (When the thermal ink jet system is employed, heat is added thereto in addition to them). The liquid droplet formation particularly on the basis of the principle of the ink jet system has the problem that storage in a long period is unstable, and further that the above-described physical force is extremely higher than the shear force and thermal energy applied in normal stirring and heat treatment. (It is considered that, for instance, the instantaneous load of 90 atmospheres at 300° C. is applied to the liquid droplets in the thermal ink jet system). In addition, a plurality of physical forces are simultaneously applied to the liquid droplets. For this reason, protein tends to become much more unstable than in a process of normally treating protein, so that there has been a case where a conventionally-used technology of stabilizing the protein is insufficient. Once the problem happens, proteins aggregate while the droplets are formed, which causes clogging in a nozzle and makes the droplets hardly ejected.
Furthermore, a size of a droplet suitable for lung inhalation is 1 to 5 μm, which is very smaller than a droplet of about 16 μm used in a currently commercially available printer, and results in applying larger surface energy and shear force onto the droplet. For this reason, it is extremely difficult to eject protein as microdroplets suitable for the lung inhalation.
In consideration of the above-described various uses, a method of ejecting a protein solution is preferably based on the principle of a thermal ink jet system, because it has a low manufacturing cost and can increase the density of nozzles.
On the other hand, a method of adding a water-soluble polymer or albumin, such as a surfactant, glycerol, various saccharides and polyethylene glycol, which are known as the method of stabilizing protein, has little or no effect of improving ejecting performance when ejecting protein with a thermal ink jet system.
In regard to a liquid composition of droplets to be inhaled into lungs, which are formed with the use of a thermal ink jet system, there is disclosed a method of adding a compound for adjusting surface tension and/or a moisturizing agent to a protein solution (for instance, the pamphlet of International Publication No. WO02/094342). The above method includes adding a water-soluble polymer such as a surfactant and polyethylene glycol to a protein solution, because describing that the water-soluble polymer improves the stability of protein in a solution of a formed liquid droplet, through decreasing surface tension and viscosity of the solution, and keeping the moisture of the solution.
However, the pamphlet does not describe the stability of ejection, furthermore, the addition of a surfactant and a water-soluble polymer shows the insufficient effect when the concentration of protein and peptide is high, and there was a case where an additive in itself aggravated the stability of the ejection. In addition, many surfactants do not have an effect on the stability of the ejection at all, and in other words, surface tension, viscosity or a moisture retention effect does not control the stability of the ejection. To put it differently, the above-described method was not a general one for stabilizing the ejection when ejecting protein and peptide with a thermal ink jet system.
As described above, an ink jet technology is well known as a method of making liquid fine droplets from a liquid sample and ejecting them, and particularly has a feature of showing high controllability for even a trace amount of a liquid to be ejected after having converted into liquid droplets. The fine-droplet-ejecting type of an ink jet system includes an oscillation type using a piezoelectric element and a thermal ink jet system using a micro-heater element. The oscillation type using the piezoelectric element has limitation in miniaturization for the piezoelectric element to be used, and accordingly in the number of installed ejection orifices per unit area. In addition, a necessary cost for producing an ejection device sharply increases with the increase of the number of arranged ejection orifices per unit area. In contrast to this, the thermal ink jet system can comparatively easily miniaturize the micro-heater elements to be used in the ejection device, can increase the number of the arranged ejection orifices per unit area in comparison with the oscillation type system using the piezoelectric element, and can far reduce a necessary production cost for the ejection device.
When applying a thermal ink jet system to liquid droplet formation, it is necessary to adjust a physical property of a liquid to be ejected, so as to control an appropriate atomisation state and a liquid volume of a liquid fine droplet to be ejected from each ejection orifice. Specifically, a liquid composition such as a type of a solvent, a composition and a concentration of a solute, which composes a liquid sample to be ejected is well adjusted so as to provide a target liquid volume of a liquid fine droplet.
Furthermore, various technological developments are being proceeded also on a droplet-ejecting mechanism based on the principle of a thermal ink jet system. While a conventional ink jet printer head ejects liquid droplets having the individual liquid volume of about several picoliters, an ejecting technology and an ejecting mechanism which have been developed recently forms an extremely liquid fine droplet of a subpicoliter or femtoliter order (see, for instance, Japanese Patent Application Laid-Open No. 2003-154655).